This invention relates to an absorbent core for absorbing liquids, such as a component of a diaper or incontinent device worn against the body.
Various designs of absorbent cores have been proposed. The absorbent material most normally used are comprised of a blend of cellulose pulp fibers and superabsorbent polymer particles covered by a nonwoven fabric. The pulp content of the core is typically 60-80 per cent of the total core weight.
The use of cellulose pulp has several disadvantages such as: a higher cost per amount of body fluid absorbed than superabsorbing polymer or "SAP" particles; slow fluid acquisition rate when the pulp fiber is compressed to form a compact absorbent article; and the high capital, productivity, and utility cost of operating a pulp mill on each and every absorbent product converting line.
The use of superabsorbent polymer in a pulp-filled core is standard throughout the absorbent product industry. These polymers swell and form a gel when contacted with aqueous liquids and are capable of acquiring and retaining under pressure many times their weight of liquid. The polymers, in powdered form, are typically disposed among pulp fibers in products such as diapers or between layers of nonwoven in products such as absorbent mats.
Since SAP particles are more cost effective than pulp fibers for retaining liquids, the primary role of pulp fibers in absorbent products has evolved from fluid retention to holding the SAP particles in place and to wicking fluid to the SAP particles. The intimate blend of SAP and pulp means that swelling SAP particles engulf the microscopic fluid wicking channels between pulp fibers. This phenomena, called "gel-blocking", prevents fluid from being wicked to many unsaturated areas of the core. Thus, only a fraction of the absorbent core absorbs fluid in use.
Various problems with the use of superabsorbing polymers in absorbent cores and some proposed solutions are described in U.S. Pat. No. 5,013,309. For example, proposals have been made to retain the polymer particles in a fixed position in a fibrous substrate to prevent migration or clumping of the particles.
Proposals have been advanced to use water soluble adhesives in a structure to prevent migration of the SAP polymer. These adhesive, however, add to the cost and processing of the product and reduce flexibility and initial porosity.
In U.S. Pat. No. 5,411,497, pockets of SAP are held between two layers secured together with a water soluble adhesive. When the core becomes wet, the bonded layers come apart, exposing additional SAP and allowing flow between layers.
Another example of an absorbent article containing SAP particles is found in U.S. Pat. No. 4,055,180, which discloses SAP disposed with a bubble film structure to prevent migration, in combination with layers of pulp. Since the plastic bubbles are relatively nonextensible. The pockets cannot swell, and the overall bulk in dry form is very large.
EP 297411 discloses a core filled with thermoplastic fibers and SAP and wrapped in tissue paper. The thermoplastic fibers are also employed to allow sealing by heat fusion, but the bulk due to the fibers is very high, and gel blockage is likely to occur.